A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that example, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ                          NA              PS                                                          (        1        )            where λ is the wavelength of the radiation used, NAPS is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NAPS or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation sources are configured to output a radiation wavelength of about 13 nm. Thus, EUV radiation sources may constitute a significant step toward achieving small features printing. Such radiation is termed extreme ultraviolet or soft x-ray, and possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings. Along with useful EUV in-band radiation, EUV radiation sources may produce almost equal (and sometimes more) undesirable out-of-band infrared (“IR”) and deep ultraviolet (“DUV”) radiation.
The source of EUV radiation is typically a plasma source, for example a laser-produced plasma or a discharge source. A common feature of any plasma source is the inherent production of fast ions and atoms, which are expelled from the plasma in all directions. These particles can be damaging to the collector and condenser mirrors which are generally multilayer mirrors, with fragile surfaces. These surfaces are gradually degraded due to the impact, or sputtering, of the particles expelled from the plasma and the lifetime of the mirrors is thus decreased. The sputtering effect may be particularly problematic for the collector mirror. The purpose of this mirror is to collect radiation which is emitted in all directions by the plasma source and direct it towards other mirrors in the illumination system. The collector mirror is positioned very close to, and in line-of-sight with, the plasma source and therefore receives a large flux of fast particles from the plasma. Other mirrors in the system are generally damaged to a lesser degree by sputtering of particles expelled from the plasma since they may be shielded to some extent.
Extreme ultraviolet (EUV) sources may use tin (Sn) or another metal vapor to produce EUV radiation. With such EUV sources, Sn may be discharged or deposited on the EUV collector. In order to achieve sufficient lifetime for the EUV lithographic apparatus, it is desirable to remove tin from the EUV collector.
Hydrogen radicals may be used to remove Sn and other contamination from optical elements, as described in United States patent application publication no. 2006/0115771. A cleaning rate greater than about 1 nm/sec with H2 radicals may be obtained when Sn contamination is deposited on a Si substrate. However, experiments on Ru substrates have shown that the cleaning rate with H2 radicals is a lot lower than on Si substrates and full cleaning of Ru substrates (i.e. all Sn removed from the Ru substrate) may not be possible.
One option that may be pursued fully to remove Sn deposition on a Ru substrate is to add a capping layer to the multi-layer mirror surface. In United States patent application publication no. 2006/0115771 describes the use of such a capping layer to protect various optical elements. With such a process, it may be possible fully to clean a multi-layer optical element with Ru-top, Mo-top or Si-top, using hydrogen plasma.